The invention relates to a projection system for making photographic exposures with a refractive projection objective. More specifically, the invention relates to the refractive projection objective itself as well as to a method involving the use of the projection system with the refractive projection objective in the manufacture of components carrying a microstructure. All lenses of the projection objective consist of the same material, and the numerical aperture of the projection objective on the image side is larger than 0.7.
The German patent application DE 198 18 444 A1 discloses refractive projection objectives that are designed for exposures with light of a wavelength of 248.4 nm, where all lenses of the projection objectives consist of a material which at the stated exposure wavelength have a refractive index of 1.50839 which is characteristic of, e.g., quartz glass.
The aforementioned reference further discloses that when image aberrations occur, they can be corrected by a targeted use of aspheres. For example, as mentioned in this reference, an image distortion occurring with the projection objective can be corrected by using an asphere in the first lens group, which in this case is a lens group of positive refractive power. Further according to the same reference, entrance pupil aberrations occurring in the projection objective can be corrected by including an asphere in the second lens group, which is a group of negative refractive power and forms a first waist of the projection objective. It is also known that by arranging an aspheric lens surface in the third lens group it is possible to minimize a coma effect that may be present in the projection objective, where the third lens group is a group of positive refractive power and is arranged between the two waists (second and fourth lens group). A coma effect occurring in the objective can likewise be minimized by arranging an asphere in the sixth lens group, which is of positive refractive power and is arranged directly in front of the wafer. Through the use of an asphere in the fifth lens group, which is of positive refractive power, it is possible to correct aberrations associated with a large numerical aperture, in particular spherical aberrations. A correction of spherical aberrations is also possible by arranging an asphere in the fourth lens group, as long as the asphere is arranged close to the image plane.
As disclosed in U.S. Pat. No. 5,668,672, chromatic aberrations can be corrected by using quartz glass in combination with a fluoride material as lens materials. Further known from U.S. Pat. No. 6,377,338 is a refractive projection objective, in which chromatic aberrations are corrected by using a combination of two or more kinds of fluoride crystals. The projection objective shown in FIG. 11 of U.S. Pat. No. 6,377,338, which is designed for a wavelength of 157 nm, includes several aspheres. The lens materials proposed for use at this wavelength include in particular calcium fluoride and lithium fluoride.
In the U.S. patent application Ser. No. 09/694,878 (EP 1094350 A), the use of individual calcium fluoride lenses is proposed for the correction of chromatic aberrations in an objective designed for the wavelength of 193 nm wherein most of the lenses consist of quartz glass. The projection objective presented in FIG. 1 of this reference is a refractive objective with a numerical aperture of 0.7 and includes a lens group of negative power providing a clearly defined waist that is identified in the drawing as G2.
A projection objective that is likewise designed for a wavelength of 193 nm is described in U.S. Pat. No. 6,522,484. This lens system has a numerical aperture of 0.7 and the specified lens materials are quartz glass and calcium fluoride used in combination. The projection objectives proposed in this reference further have at least two lens groups of negative power, each of which produces a clearly defined waist in the light path geometry.
Refractive lens systems are described in EP 1139138 A1, in which the lenses consist of the materials calcium fluoride and quartz glass. An example of an objective designed for a wavelength of 157 nm is shown in which all lenses consist of calcium fluoride. Other lens arrangements presented in the same reference are designed for the wavelength of 193 nm. Each of the lens systems described includes a plurality of aspheres.
Using calcium fluoride, e.g., in a lens system designed for exposures at a wavelength of 193 nm has the disadvantages that on the one hand calcium fluoride is not as readily available as quartz glass and on the other hand it is also significantly more expensive.